1. Field of the Invention
This invention relates generally to systems for detection of objects having preselected properties and more particularly to the class of detection systems used for prevention of merchandise theft from a protected area. A marker tag is attached to an article of merchandise for which protection is sought. This marker tag can be rendered inoperative by an operator for a permitted removal of an article from the protected area. Apparatus, located at an exit station of the protected area, detects the passage of any marker tag which has not been rendered inoperative and therefore identifies an associated protected article of merchandise for which removal is unpermitted.
2. Description of the Prior Art
In the prior art, methods are known for providing detection apparatus responsive to the passage of a marker tag through a specified location. In typical operation, an article of merchandise over which control is sought in a protected area, has attached thereto a marker tag. Upon purchase, the marker tag is either rendered inoperative, removed from the article of merchandise, or routed in a manner to avoid the detection apparatus. A detection of a tag material typically indicates an unauthorized removal of the attached merchandise from the protected area.
There are, however, many problems associated with the prior attempts to control articles of merchandise using this type of system. One important problem is the ability to discriminate between the marker tag and the multiplicity of other objects which a person, with access to the protected area, might possess. Examples of the diversity range from carriages for infants to soda cans. This discrimination is particularly important to permit as much freedom of access as possible to a protected area while minimizing the compromise of goodwill associated with mistaken identification of another article as a marker tag.
A further problem with prior systems involves the existence of system "blind spots." "Blind spots" result when the detection apparatus is unable to detect the presence of a marker tag in certain regions of an exit station or certain orientations of the marker tag. The problem arises because the geometry of a tag material unavoidably enters into the detection process, and can also occur in some cases because a person's body is capable of shielding the tag from the interrogation fields used by certain systems.
Still another problem with prior systems involves "masking" in a system by the introduction of various metallic objects. These objects can be electrically conductive or can have magnetic properties, while certain materials can have both electrical and magnetic properties. These objects, although possibly not resulting in activation of the detection apparatus, can produce large field disturbances and associated burdens on the detection apparatus, such that the presence of a marker tag cannot be detected and is therefore masked.
A further problem with prior systems involves the size of the marker tags. In order to be inconspicuous when attached to an article of merchandise, it is desirable that the tag be physically as small as possible. This size requirement is in conflict with the need for the tag to produce a large effect to activate the detection apparatus.
It is therefore an object of the present invention to provide an improved theft detection system.
It is another object of the present invention to provide a theft detection system possessing the capability of disciminating between a marker tag and other materials producing detectable field perturbation effects.
It is still another object of the present invention to provide apparatus for minimizing the required dimensions of a marker tag.
It is a more particular object of the present invention to provide a marker tag containing magnetic material and apparatus for cycling the tag material through regions of differing permeability.
It is another particular object of the present invention to provide apparatus for electrically varying the spatial orientation of the field used in the detection of the magnetic marker tag.
It is yet another object of the present invention to subject a magnetic material to an applied magnetic field having two different frequency components, the marker tag producing a component with a new frequency not contained in the two applied fields or linear combinations thereof.
It is a more specific object of the present invention to detect the amplitude and phase of a new magnetic field component of a third frequency produced by the influence of the low loss magnetic marker tag material on the applied magnetic field.
It is an even more particular object of the present invention to detect a unique third frequency electromagnetic field component, generated by a marker tag in response to a spatially varying applied field, independent of the spatial orientation of a marker tag.